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Abstract:

A method for producing a photovoltaic module by contacting at least one
layer of liquid encapsulant and a plurality of solar cells. The liquid
encapsulant has two components. The first component is an acrylic polyol
having a terminal hydroxy group, an average number of hydroxy-functional
monomer units per polymer chain from 2 to 25 and Mn from 1,000 to 10,000.
The second component is an aliphatic polyisocyanate with an average
functionality of at least two. The molar ratio of non-terminal hydroxy
groups in the polyol to isocyanate groups in the aliphatic polyisocyanate
is from 0.5:1 to 1:0.5.

Claims:

1. A method for producing a photovoltaic module; said method comprising
contacting at least one layer of liquid encapsulant and a plurality of
solar cells; wherein the liquid encapsulant comprises: (i) an acrylic
polyol having a terminal hydroxy group, an average number of
hydroxy-functional monomer units per polymer chain from 2 to 25 and Mn
from 1,000 to 10,000; and (ii) an aliphatic polyisocyanate with an
average functionality of at least two; wherein a molar ratio of
non-terminal hydroxy groups in the polyol to isocyanate groups in the
aliphatic polyisocyanate is from 0.5:1 to 1:0.5.

2. The method of claim 1 in which the acrylic polyol has an average
number of hydroxy-functional monomer units per polymer chain from 3 to 6.

3. The method of claim 1 in which the acrylic polyol has Mn from 3,000 to
7,000.

4. The method of claim 1 in which the aliphatic polyisocyanate has an
average functionality from 2 to 4.

5. The method of claim 1 in which the acrylic polyol comprises from 65 to
95 wt % polymerized residues of C4-C12 alkyl(meth)acrylate and
5 to 35 wt % polymerized residues of hydroxy-containing acrylic monomer.

6. The method of claim 5 in which the acrylic polyol has Mn from 3,000 to
5,000 and from 10 to 20% residues of hydroxy-containing acrylic monomer.

7. A composition comprising: (i) an acrylic polyol having a terminal
hydroxy group, an average number of hydroxy-functional monomer units per
polymer chain from 2 to 25 and Mn from 1,000 to 10,000; and (ii) an
aliphatic polyisocyanate with an average functionality of at least two;
wherein a molar ratio of non-terminal hydroxy groups in the polyol to
isocyanate groups in the aliphatic polyisocyanate is from 0.5:1 to 1:0.5.

8. The composition of claim 6 in which the acrylic polyol has an average
hydroxyl functionality per polymer chain from 3 to 6.

9. The composition of claim 8 in which the acrylic polyol comprises from
65 to 95 wt % polymerized residues of C4-C12
alkyl(meth)acrylate and 5 to 35 wt % polymerized residues of
hydroxy-containing acrylic monomer.

10. The composition of claim 9 in which the acrylic polyol has Mn from
3,000 to 5,000 and from 10 to 20% residues of hydroxy-containing acrylic
monomer.

Description:

[0002] This invention relates to a liquid encapsulant particularly useful
for construction of photovoltaic modules and to a process for producing a
photovoltaic module.

[0003] Methods have been disclosed for encapsulation of solar cells into a
photovoltaic module. For example, U.S. Pub. No. 2006/0207646 discloses a
process using a liquid silicone encapsulant. However, use of
acrylic-urethane resins for this purpose has not been reported.

[0004] The problem addressed by the present invention is to provide a
liquid encapsulant particularly useful for construction of photovoltaic
modules and a process for producing a photovoltaic module.

STATEMENT OF INVENTION

[0005] The present invention provides a method for producing a
photovoltaic module comprising contacting at least one layer of liquid
encapsulant and a plurality of solar cells; wherein the liquid
encapsulant comprises: (i) an acrylic polyol having a terminal hydroxy
group, an average number of hydroxy-functional monomer units per polymer
chain from 2 to 25 and Mn from 1,000 to 10,000; and (ii) an aliphatic
polyisocyanate with an average functionality of at least two; wherein a
molar ratio of non-terminal hydroxy groups in the polyol to isocyanate
groups in the aliphatic polyisocyanate is from 0.5:1 to 1:0.5.

[0006] The present invention is further directed to a composition
comprising: (i) an acrylic polyol having a terminal hydroxy group, an
average number of hydroxy-functional monomer units per polymer chain from
2 to 25 and Mn from 1,000 to 10,000; and (ii) an aliphatic polyisocyanate
with an average functionality of at least two; wherein a molar ratio of
non-terminal hydroxy groups in the polyol to isocyanate groups in the
aliphatic polyisocyanate is from 0.5:1 to 1:0.5.

[0008] The term "vinyl monomers" refers to monomers that contain a
carbon-carbon double bond that is connected to a heteroatom such as
nitrogen or oxygen. Examples of vinyl monomers include, but are not
limited to, vinyl acetate, vinyl formamide, vinyl acetamide, vinyl
pyrrolidone, vinyl caprolactam, and long chain vinyl alkanoates such as
vinyl neodecanoate, and vinyl stearate.

[0009] A solar cell is a semiconductor used to generate electricity from
light. Solar cells typically are made from semiconductor materials such
as silicon (crystalline, polycrystalline or thin film), gallium arsenide,
copper indium diselenide, cadmium telluride, copper indium gallium
diselenide, and mixtures thereof. Solar cells may be in the form of
wafers or thin films, the former being made by cutting from a crystal or
casting and the latter deposited on a substrate or superstrate by
sputtering or chemical vapor deposition (CVD).

[0010] The average number of hydroxy-functional monomer units per polymer
chain is an average value calculated for the acrylic polyol from its
values of Mn and equivalent weight (EW) (units/chain=Mn/EW). The
equivalent weight is defined as the mass of polyol which contains one
mole of hydroxyl functionality, excluding hydroxy end groups. For
example, a polyol containing 15 wt % HEMA has EW=876.6 g polyol/mole OH.
If the hydroxyl number (OH#) has been determined for the polyol, then the
calculation is as follows: Mn/(56105/OH#). The hydroxyl number is
calculated from the hydroxy-functional monomer content of the polymer,
without including the hydroxy end group believed to be derived from the
chain transfer agent. OH#=56105/EW. The actual distribution of polymer
chains will of course contain some chains with lower and higher hydroxyl
functionality. In some embodiments of the invention, the average hydroxyl
functionality per polymer chain is at least 2.5, preferably at least 3,
preferably at least 3.5, preferably at least 4; the average hydroxyl
functionality is no greater than 10, preferably no greater than 8,
preferably no greater than 7, preferably no greater than 6.

[0011] In some embodiments of the invention, Mn of the acrylic polyol is a
least 2,000, preferably at least 2,500, preferably at least 3,000,
preferably at least 3,500. In some embodiments of the invention, Mn of
the acrylic polyol is no greater than 8,000, preferably no greater than
7,000, preferably no greater than 6,000. In some embodiments Mw/Mn is
from 1.5 to 3.5, alternatively from 2 to 3. In some embodiments of the
invention, the Tg of the acrylic polyol is from -100° C. to
-40° C., preferably from -80° C. to -45° C.,
preferably from -75° C. to -50° C.

[0012] In some embodiments of the invention, the acrylic polyol comprises
at least 60% polymerized residues of acrylic monomers, preferably at
least 70%, preferably at least 80%, preferably at least 90%, preferably
at least 95%. In some embodiments of the invention, the acrylic polyol
contains from 5 to 35% polymerized residues of hydroxy-containing acrylic
monomers, preferably from 8 to 25%, preferably from 10 to 20%. In some
embodiments of the invention, the acrylic polyol has Mn from 3,000 to
5,000 and from 10 to 20% residues of hydroxy-containing acrylic monomers.
In some embodiments, hydroxy-containing monomers are
hydroxyalkyl(meth)acrylates, preferably those selected from HEMA, HPMA,
HBA or combinations thereof; preferably HEMA and/or HPMA. In some
embodiments, the acrylic polyol comprises from 65 to 95% polymerized
residues of C4-C12 alkyl (meth)acrylate(s), preferably from 75
to 92%, preferably from 80 to 90%; in some embodiments, the
C4-C12 alkyl(meth)acrylate(s) are C4-C12 alkyl
acrylate(s), preferably C4-C10 alkyl acrylate(s), preferably BA
and/or EHA. In some embodiments, the acrylic polyol may contain small
amounts of residues of vinyl monomers in addition to acrylic monomers.

[0013] Preferably, the acrylic polyol is made by a solution polymerization
using typical initiators well known in the art. Preferably, a chain
transfer agent (CTA) is used, e.g., an alcohol, glycol, glycol alkyl
ether, mercapto-alcohol or mercapto-glycol; preferably an alcohol, glycol
or glycol alkyl ether; preferably an alcohol. In some embodiments, the
chain transfer agent is substantially free (i.e., less than 0.3%,
alternatively less than 0.1%, alternatively less than 0.05%) of sulfur
and the acrylic polyol is substantially free (i.e., less than 100 ppm,
alternatively less than 50 ppm, alternatively less than 25 ppm) of
sulfur. Suitable solvents for the polymerization include, e.g., alcohols,
alkyl esters, glycols, glycol alkyl ethers, aldehydes, ketones and
ethers. In some embodiments, the solvent also acts as the chain transfer
agent; preferred solvents which are also chain transfer agents include,
e.g., C1-C6 alcohols, including isopropanol. When hydroxy
compounds are used as chain transfer agents, a terminal hydroxy group is
believed to be attached directly to the end of the polymer chain. When
alcohols, e.g., isopropanol, are used as chain transfer agents, the
resulting tertiary terminal hydroxy group is believed to be chemically
less reactive than hydroxy groups on hydroxyalkyl(meth)acrylates and
other hydroxy-substituted monomers.

[0014] An aliphatic polyisocyanate is a material having an average
isocyanate functionality of at least 2. Examples of suitable aliphatic
polyisocyanates include those based on isophorone diisocyanate (IPDI),
hexamethylene diisocyanate (HDI), dicyclohexyl methane diisocyanate
(HMDI), bis(isocyanatomethyl)cyclohexane, isomers thereof or mixtures
thereof. Prepolymers of an aliphatic polyisocyanate and a polyol may also
be used in this invention as the aliphatic polyisocyanate component;
preferred Mn for a polyisocyanate prepolymer is from 300 to 3000,
preferably from 500 to 2000. In some embodiments of the invention, the
functionality of the aliphatic polyisocyanate is at least 2.5,
alternatively at least 2.7, alternatively at least 3. In some
embodiments, the aliphatic polyisocyanate has functionality no greater
than 5, preferably no greater than 4, preferably no greater than 3.

[0015] Preferably, the molar ratio of non-terminal hydroxy groups/NCO
groups varies from 0.75:1 to 1:0.75, alternatively from 0.75:1 to 1:0.9,
alternatively from 0.9:1 to 1:0.75, alternatively from 0.9:1 to 1:0.9,
alternatively from 0.95:1 to 1:0.9, alternatively from 0.9:1 to 1:0.95,
alternatively from 0.95:1 to 1:0.95, alternatively 0.98:1 to 1:0.98,
alternatively 0.99:1 to 1:0.99, alternatively 0.995:1 to 1:0.995.

[0016] Crosslinkers are monomers having two or more ethylenically
unsaturated groups, and may include, e.g., divinylaromatic compounds,
di-, tri- and tetra-(meth)acrylate esters, di-, tri- and tetra-allyl
ether or ester compounds and allyl(meth)acrylate. Preferred examples of
such monomers include divinylbenzene (DVB), trimethylolpropane diallyl
ether, tetraallyl pentaerythritol, triallyl pentaerythritol, diallyl
pentaerythritol, diallyl phthalate, diallyl maleate, triallyl cyanurate,
Bisphenol A diallyl ether, allyl sucroses, methylene bisacrylamide,
trimethylolpropane triacrylate, allyl methacrylate (ALMA), ethylene
glycol dimethacrylate (EGDMA), hexane-1,6-diol diacrylate (HDDA) and
butylene glycol dimethacrylate (BGDMA). In some embodiments of the
invention, the amount of polymerized crosslinker residue in the polymer
is no more than 0.2%, preferably no more than 0.1%, preferably no more
than 0.05%, preferably no more than 0.02%.

[0017] The composition of the present invention optionally may include
other ingredients. For example, the composition may include catalyst for
the urethane forming reaction, e.g., dialkyl tin diesters; adhesion
promoters; antioxidants and light stabilizers. The aforementioned
ingredients preferably would be in the polyol component, although some
may be included in the polyisocyanate component if they are not reactive
with isocyanate groups and do not catalyze isocyanate polymerization. In
some embodiments of the invention, the composition contains no more than
1.5 wt % light stabilizer(s), alternatively no more than 1.25%,
alternatively no more than 1%, alternatively no more than 0.75%,
alternatively no more than 0.5%, alternatively no more than 0.25%. In
some embodiments of the invention, the photovoltaic module is formed
using only the encapsulant and the solar cells to form a cured
encapsulant in which the solar cells are suspended. In some embodiments
of the invention, the photovoltaic module is formed by laminating two
flexible sheets of material together using the liquid encapsulant; in
this case the cells may be formed either on the bottom side of the top
film, or the top side of the bottom film. In some further embodiments of
the invention, these two flexible films are laminated in a roll-to-roll
process. In some embodiments of the invention, a glass sheet covers the
solar cells to form a photovoltaic module in which light passes through
the glass sheet before striking the solar cells. In these embodiments,
the solar cells may be separated from the glass by a layer of cured
encapsulant material or the solar cells may be formed directly on the
glass sheet (e.g., by sputtering or CVD) and then covered with a layer of
the encapsulant material. When the solar cells are separated from the
glass by a layer of cured encapsulant material, they may be formed on top
of a solid sheet, e.g., of metal foil or glass, then covered with a layer
of liquid encapsulant, and finally with a glass sheet; alternatively, the
solar cells may be located between two layers of encapsulant with the
glass on top, and an optional layer of rigid material below the layers of
encapsulant. The rigid material may be, e.g., glass, a synthetic polymer
(e.g., polyvinyl fluoride, polyethylene terephthalate, ethylene vinyl
acetate), a metal sheet, etc. In these descriptions, the term "top"
indicates the direction from which light travels to reach the solar
cells. In most cases, the photovoltaic module requires an insulating
material below the solar cells. This material may be the encapsulant or a
rigid material, provided that the material meets criteria for insulating
ability, e.g., test methods specified in IEC 61215, IEC 61646, UL 746A,
UL746B, UL 746C.

[0018] In some embodiments of the invention, the polyol component and the
isocyanate component are mixed to form the liquid encapsulant just prior
to contact with the solar cells, e.g., in an in-line mixer or in a mixing
tank. Depending on the type of construction of the photovoltaic module,
as described above, the mixed components could be applied to glass or a
synthetic polymer material prior to adding the solar cells, prior to and
after the solar cells to form two layers of encapsulant material, to
solar cells formed on glass or a synthetic polymer, etc. Preferably, the
liquid encapsulant is cured by heating the assembled photovoltaic module,
preferably to a temperature from 60° C. to 150° C. for a
time from 1 minute to 3 hours. Times and temperatures will vary depending
on the types of isocyanate and hydroxy functionality, hydroxy/isocyanate
ratio and other factors, as is well understood in this field.

EXAMPLES

Typical Polymerization Procedure

[0019] Isopropanol (1137 g) was charged to a 4-neck, round-bottom flask
fitted with nitrogen purge, reflux condenser, temperature controller, and
mechanical stirrer, and heated to 82° C. A solution of t-amyl
peroxypivalate in isopropanol was added (704 mL, 4.7 wt %), followed by a
mixture of 2-ethylhexyl acrylate (187 g) and hydroxyethyl methacrylate
(33 g). The temperature of the mixture increased to 86° C. without
added heat. After allowing the mixture to cool to 82° C., a
mixture of 2-ethylhexyl acrylate (1683 g) and hydroxyethyl methacrylate
(297 g) was gradually added over a period of 180 minutes. Concurrently, a
solution of t-amyl peroxypivalate in isopropanol (704 mL, 4.7 wt %) was
added over a period of 200 minutes. After complete addition, the mixture
was maintained at 82° C. for 60 minutes, followed by addition of a
solution of t-amyl peroxypivalate in isopropanol (16.6 mL, 29.8 wt %).
After 15 minutes, another solution of t-amyl peroxypivalate in
isopropanol (16.6 mL, 29.8 wt %) was added over period of 90 minutes. The
solvent was removed from the polymer by evaporation.

[0020] The following table lists polymers made according to this method.

[0021] This procedure is for an EHA/HEMA 85/15 copolymer crosslinked with
a stoichiometric amount of HMDI (1:1 hydroxy/isocyanate molar ratio,
based on hydroxy functionality of polyol excluding hydroxy end group).
The copolymer (50 g, OH# 64.5 mg KOH/g), catalyst (dibutyltin diacetate,
0.005 wt %, 0.0025 g), and optional photostabilizer were mixed under
vacuum at 60° C. until no bubbles were observed.
Dicyclohexylmethane-4,4'-diisocyanate (7.62 g) was added and the
formulation was mixed under vacuum for 3 minutes. The first three (or
two, if no photopackage) ingredients were degassed at 60° C., and
the isocyanate was pumped separately at a stoichiometric ratio into an
in-line static mixer with 24 mixing elements.

1) Glass Down, Precure

[0022] The mixture described above was coated onto a 6''×6''glass
plate using a drawdown bar, resulting in wet coating thickness of 500
microns. This plate was placed in an oven at 90C for 1 hour.
Subsequently, a second coating of the liquid encapsulant was made over
this partially cured layer. A 5'' (12.7 cm) single crystal silicon solar
cell with front and rear bus ribbons was placed face down on the wet
encapsulant, and a third coating made over the backside of the solar
cell. An EVA/PET/TEDLAR backsheet film was placed EVA side down on the
wet encapsulant, and any air bubbles removed using a rubber roller. The
laid-up solar module was placed in an oven and cured overnight at 90C.

2) Glass Down, No Precure

[0023] The mixture described above was coated onto a 6''×6''
(15.2×15.2 cm) glass plate using a drawdown bar. A 5'' (12.7 cm)
single crystal silicon solar cell with front and rear bus ribbons was
placed face down on the wet encapsulant, and a second coating of liquid
encapsulant made over the backside of the solar cell. An EVA/PET/TEDLAR
backsheet film was placed on the wet encapsulant, and any air bubbles
removed using a rubber roller. The laid-up solar module was placed in an
oven and cured overnight at 90C. In several modules, the backsheet was
found to have partially slid off the glass.

3) Glass Down, No Precure; Fixture

[0024] The mixture described above was coated onto a 6''×6''
(15.2×15.2 cm) glass plate using a drawdown bar. This glass plate
was placed into an aluminum fixture which is sized to fit the glass and
backsheet. The parts of the fixture which touch the workpiece are treated
with fluoropolymer release agent. A 5'' (12.7 cm) single crystal silicon
solar cell with front and rear bus ribbons was placed face down on the
wet encapsulant, and a second coating of liquid encapsulant made over the
backside of the solar cell. An EVA/PET/TEDLAR backsheet film was placed
on the wet encapsulant, and any air bubbles removed using a rubber
roller. The laid-up solar module was placed in an oven and cured
overnight at 90C. After curing, the module was removed from the aluminum
fixture.

4) Glass Up, No Precure, Fixture

[0025] The mixture described above was coated onto a 6''×6''
(15.2×15.2 cm) piece of EVA/PET/TEDLAR backsheet film using a
drawdown bar. A 5'' (12.7 cm) single crystal silicon solar cell with
front and rear bus ribbons was placed face up on the wet encapsulant. A
second layer of encapsulant was coated over the solar cell, and a
6''×6'' (15.2×15.2 cm) piece of glass laid on top slowly from
one edge so no bubbles were trapped. This layup was transferred to an
aluminum fixture, and the assembly cured in an oven overnight at 90C.
After curing, the module was removed from the aluminum fixture.

5) Predicted Procedure for Hot Glass

[0026] The mixture described above is coated onto a 6''×6''
(15.2×15.2 cm) piece of glass which is preheated to 12° C. A
5'' (12.7 cm) single crystal silicon solar cell with front and rear bus
ribbons is placed face down on the wet encapsulant, and a second coating
of liquid encapsulant is made over the backside of the solar cell. An
EVA/PET/TEDLAR backsheet film is placed on the wet encapsulant, and any
air bubbles removed using a rubber roller. The laid-up solar module is
placed in an oven and cured overnight at 90C.

6) Predicted Procedure for Use of Frame as Fixture

[0027] The mixture described above is coated onto a 6''×6''
(15.2×15.2 cm) glass plate using a drawdown bar. A 5'' (12.7 cm)
single crystal silicon solar cell with front and rear bus ribbons is
placed face down on the wet encapsulant, and a second coating of liquid
encapsulant made over the backside of the solar cell. An EVA/PET/TEDLAR
backsheet film with openings cut for the egress of the bus ribbons was
placed on the wet encapsulant, and any air bubbles removed using a rubber
roller. The laid-up solar module was placed into an aluminum frame, and
additional mixture added to the edge of the frame to seal the frame to
the module. The framed module is then placed in an oven and cured
overnight at 90C.

[0028] The following table lists test results from photovoltaic modules
made without photostabilizer according to the above procedure. The polyol
made using IPA as CTA is believed to have a hydroxy end group.
Encapsulant polymer made from this polyol and HMDI exhibited much less
yellowing than the comparison polymer. Yellowing was measured after the
exposure to a Xe arc lamp for the indicated times, according to ASTM G
155.